Reef sediment study

This project is now complete.

Natural Resources Adelaide and Mount Lofty Ranges (AMLR), in partnership with South Australian Research and Development Institute (SARDI) – Aquatic Sciences, has studied sedimentation on Adelaide’s coastal reefs.

Run-off from rivers, rain and discharge from stormwater wash soil, nutrients and other particles down to the coast. These particles eventually settle to form layers or sediments. Sediments can also come from many other sources including waste water such as sewage treatment plants and industry discharge.

Sedimentation can cause many problems to the marine environment because:

particles cause cloudiness, or turbidity, in the water, which can reduce light and may affect photosynthesis of marine plants and reduce feeding of other organisms

filter feeding animals, such as some invertebrates, may be unable to extract their food from water due to an increase in sediments

a thick layer of sediment may slow larval development of some marine plants and animals that require hard substrates, such as reefs, to attach to and grow.

Sedimentation has been identified as a threat to the region’s reef systems. Undertaking this study has helped us understand the main sources contributing to reef sedimentation and their impact on marine plants and animals.

How was the study done?

Twelve reef systems between Semaphore and Aldinga were surveyed over summer and winter to identify areas where reefs were impacted by sediment.

What was found?

Initial results indicated that reefs close to the mouth of the Onkaparinga River receive the highest sediment loads along the metropolitan coast. Sediments from the river were observed as far south as Aldinga, although sedimentation rates drastically decrease south of Southport. The cliffs at the mouth of the river provide another source of sediments to coastal ecosystems.

Field River and Christies Creek were major transmitters of sediments to reefs between Port Noarlunga North and Hallett Cove.

In contrast, reefs in the northern section of the metropolitan coast, which are located further offshore than their southern counterparts, had relatively low sedimentation rates and showed no evidence of sediments from rivers. Sediments on these reefs, however, had high nitrogen contents and isotopic signatures suggesting they may have come from wastewater or industrial sources.

What is being done?

Information derived from the survey helps the AMLR Natural Resources Management (NRM) Board prioritise on-ground works and other actions that help reduce sediment and the potential impacts on reef health, in line with the AMLR NRM Plan. To ensure our work in improving coast and marine water quality and the condition of habitats is progressing well, monitoring is undertaken, with outcomes documented in report cards.

While small flows of water containing sediments can be more easily managed or stopped, large volumes of water and associated sediment during a flood event after a major storm can be more difficult to control and manage. Managing rivers and land condition upstream helps to prevent erosion and reduce the amount of sediment that enters rivers and creeks.

Partners

South Australian Research and Development Institute – Aquatic Sciences